1. Field of the Invention
The present invention generally relates to an external rotor motor and, more particularly, to an external rotor motor able to avoid the formation of an electrically conductive route between the core and the shaft.
2. Description of the Related Art
Referring to FIG. 1, a conventional external rotor motor 8 is shown. This external rotor motor 8 roughly includes a stator 81 and a rotor 82. The stator 81 includes a core 811 wired by a coil 812, and a shaft 813 is arranged in the center of the core 811. The rotor 82 has an upper housing 821, a lower housing 822 and a middle ring 823. The middle ring 823 links and is between the upper housing 821 and the lower housing 822. Each of the upper and lower housings 821, 822 has a bearing 824 for the shaft 813 to pass through, so that the core 811 can be positioned between the upper and lower housings 821, 822. An embodiment of said external rotor motor 8 has been disclosed by Taiwan patent no. 1326959, entitled “Improvement of Stator and Rotor of DC Brushless Motor for Ceiling Fan.”
The core 811 of the above external rotor motor 8 is directly coupled with an outer periphery 813a of the shaft 813 by press fit, and, thus, the core 811 can firmly mount on the shaft 813. However, there are power wires inside the shaft 813 usually, while the core 811 is made by silicon steel sheets that are conductive material, and, thus, electromagnetic interference to the core 811 is easily induced when there are currents in the power wires inside the shaft 813. As a result, the external rotor motor 8 may produce noise or vibration in operation, thus, further resulting in negative effects to the operational stability of the external rotor motor 8.
Moreover, since the core 811 is made of conductive material such as silicon steel sheets and the shaft 813 is made of metal, the core 811 and the shaft 813 may jointly form a conductive circuit once the electrical insulation coating of the coil 812 outside the core 811 is damaged and exposes the conductive material therein. Therefore, under the above situation, electric leakage can be easily caused by the exposed conductive material of the coil 812, and the leakage current from the coil 812 will be transferred to the outside of the external rotor motor 8 through the core 811 and the shaft 813. As a result, the usage safety of the external rotor motor 8 may largely decrease.
Please further refer to another conventional external rotor motor 9 shown by FIG. 2. This conventional external rotor motor 9 briefly includes a stator unit 91, a rotor unit 92 and a control unit 93. The stator unit 91 also has a core 911, a coil 912 wiring the core 911, and a shaft 913 arranged in the center of the core 911. The rotor unit 92 is rotatably arranged around the shaft 913, and includes an upper housing 921, a lower housing 922 and a middle ring 923 linking and between the upper housing 921 and the lower housing 922. The upper housing 921 and the lower housing 922 are arranged on two sides of the core 911, with the upper and lower housings 921, 922 having bearings 924 for the shaft 913 to pass through, to position the core 911 between the upper and lower housings 921, 922. The control unit 93 connects to a side of the core 911. The control unit 93 has a circuit board 931 and a shield 932, and the circuit board 931 is fixed inside the shield 932. An embodiment of said external rotor motor 9 has been disclosed by Taiwan patent no. M354002, entitled “Ceiling Fan.”
The core 911 of the above external rotor motor 9 is also coupled with an outer periphery 913a of the shaft 913 by press fit. Therefore, the external rotor motor 9 may also be affected by electromagnetic interference easily, thus, further leading to bad operational stability. Specifically, since the core 911 and the shaft 913 may jointly form a conductive circuit, and since the control unit 93 on the side of the core 911 will be located in the route of the conductive circuit formed by the core 911 and the shaft 913 when the leakage current from the coil 912 is transferred by the core 911 and the shaft 913, the circuit board 931 of the control unit 93 may easily be affected by the leakage current. Therefore, the elements on the board 931 may be damaged, thus leading to a failure of the conventional external rotor motor 9.
In sum, there are drawbacks such as “bad operational stability” and “low usage safety” in the external rotor motors 8, 9, and thus, further improvements to external rotor motors are necessary to improve the market value of conventional external rotor motors.